Polyethylene terephthalate (hereinafter may be referred to as “PET”) as polyester fibers has been being studied as major synthetic fibers owing to its high tenacity, good dyeability and productivity. PET fibers are useful not only for clothing but also for motor vehicles and have been mainly developed for car seats and ceiling materials. PET fibers are stable in the behavior after heat treatment, and in the heat treatment step as the final step for obtaining a woven fabric or knitted fabric (may be referred as “woven or knitted fabric” collectively), the fabric can be easily kept within the designed fabric width and little changes thereafter. Therefore, a fabric with stable quality can be obtained. Further, for the reason that fabrics that are raised for use as car seats, ceiling materials and the like look luxurious, fabrics are sometimes raised after heat treatment. Also in the raising treatment, PET fibers are likely to have stable quality.
Meanwhile, in the case where PET fibers are used as a car seat, there is a problem that the fabric remains drawn since it is repetitively loaded with human bodies. The reason is that the elastic recoverability of PET fibers is small, that is, the recovery rate after elongation is low.
Furthermore, since the initial tensile resistivity (also may be referred to as “Young's modulus” or “elastic modulus”) of PET fibers is as high as about 90 cN/dtex, especially a raised fabric formed of them has a problem that it has a stinging hardness.
On the other hand, fibers formed of polytrimethylene terephthalate (hereinafter may be referred to as “3GT”) are high in the elastic recoverability and low in the initial tensile resistivity and therefore have a feature of excellent softness. Since they have dyeability in addition, they are energetically studied in recent years as attractive polyester fibers capable of compensating the disadvantages of PET fibers.
However, 3GT fibers are not almighty either and also have disadvantages. To compensate the disadvantages, active studies are being made on 3GT fibers. For example, 3GT fibers have low tenacity, too low elastic modulus undesirable for some applications and low color fastness, and to compensate these disadvantages, core-sheath conjugate fibers using 3GT as the sheath component and PET as the core component are proposed (Patent Document 1). According to the document, core-sheath conjugate fibers with a tenacity of 3.9 to 4.7 g/d (3.5 to 4.2 cN/dtex) and an elastic modulus of 43 to 72 g/d (39 to 65 cN/dtex) can be obtained. This technique can certainly achieve a high tenacity compared with the conventional 3GT fibers but impairs the low elastic modulus that is the most important feature of 3GT fibers, causing the problem of losing softness. Further, the importantly attractive elastic recoverability of 3GT fibers greatly declines and only an unsatisfactory car seat can be obtained.
On the other hand, in the case where 3GT fibers are used as a car seat, if the state of tension is released after finish thermosetting, the seat gradually shrinks in the width direction because of the poor thermosetting property thereof, resulting in irregular quality in the transverse direction. Especially this trend is remarkable with a knitted fabric low in the force of constraining the fabric, and after the lapse of several days subsequent to thermosetting, the surface smoothness is lost to cause the problem of losing softness or the edges are curled to raise the problem of not allowing practical use.
Publicly known techniques for improving the problem of shrinkage of 3GT fibers include, for example, Patent Document 2. The document proposes to control thermal stress for the purpose of lowering the high shrinkage percentage of 3GT fibers for thereby further enhancing softness. The method of control is to spin at a high speed and to wind without heat treatment. However, this technique has been found to cause gradual shrinkage with the lapse of time, showing a poor thermosetting property, though the thermal shrinkage percentage and stress are low. The reason is that crystallization takes place gradually even after production of fibers because the crystallinity of the fibers obtained by high speed spinning is low and because 3GT has a low glass transition temperature.
As described above, fibers capable of providing a fabric suitable for a car seat have not been proposed so far.    Patent Document 1: JP11-93021 A (claims, paragraph [0011], examples)    Patent Document 2: JP2001-348729A (claims, examples)